How to add custom code to enable a popup on page load?

I still would prefer to have a better understanding of how to add custom code, it’s really unclear how things can be created locally and added to a Plasmic page. Right now, I’d like to add a popup on page load… this was asked in Plasmic forums nearly a year ago without a single response… Not sure how I can go about doing this, or if I’m starting to reach a point I need to recreate everything from scratch for lack of being able to further build and customize.

Could you use an off-the-shelf modal component (say, from antd), register it, and add it to your page?